The present invention relates to a casing structure for accommodating an automatic transmission and, more particularly, to a transaxle casing structure for a transverse type automatic transmission.
As is well known in the art, an automatic transmission employs a plurality of sets of planetary gears each having three elements, one of which is locked whereas the other two elements are used to act as input and output members to establish a variety of speedchange gear ratios. These three elements of the planetary gear sets are locked or engaged by frictional engaging means including clutches and brakes generally of wet type. As a result, the automatic transmission has a number of portions to be lubricated. Especially in the transverse type automatic transmission, as disclosed in Japanese Patent Publication No. 58 - 17896, the members such as the clutches and brakes are arranged as concentrically with the planetary gear set as possible so as to meet the requirement for shortening the total length of the transmission. This concentric arrangement discretely scatters the portions to be lubricated in the radial direction, and these portions are forcibly lubricated by a pump pressure or a centrifugal force. The lubricant having lubricated the tooth surfaces of the gears and the friction members of the wet type multi-disc clutches finally flow down into the oil sump. However, the following problems are caused by the recent tendency of the higher rotations of an engine.
The lubricant having lubricated the planetary gear sets and the wet-type multi-disc clutches is gradually caused to flow radially outwards by the centrifugal force until it is splashed away the rotary members. As a result, the lubricant having been caught by the inner wall of the casing drops into the oil sump so that it may be fed again to the planetary gear sets and so on.
If these circulating cycles are merely repeated, the lubricant is gradually heated to a higher temperature and stirred to form bubbles. These tendencies are the more serious for the more number of revolutions of the engine, i.e., the input to the automatic transmission. As a result, the lubricant level in the oil sump is raised by the volumetirc expansion due to both the rise in the oil temperature and the bubbling due to the stirrig operation. This in turn further increases the stirring of the lubricant. These vicious cycles are repeated to invite problems such as the power loss or the shortenings of the lifetimes of the lubricant itself and the automatic transmission as a whole.
In the case of a high-speed type engine, the centrigual force to be exerted upon the lubricant in the automatic transmission is strengthened to scatter more lubricant so that the lubricant frequently drops again to the rotary members once it has been caught by the inner wall of the casing. As a result, the lubricant cooling rate drops to seriously increase the oil temperature and volumetric expansion so that the pressure in the casing is raised to the higher level. Thus, the lubricant is scattered the more by the centrigual force and is probably blown away the breather.